1. Technical Field
The present invention generally relates to methods and apparatus for processing received wireless communication signals, and particularly relates to generating soft symbols from a traffic channel signal using combining weights derived from pilot channel propagation channel estimates, such that the resulting soft symbols do not vary with changes in the pilot-channel-to-traffic-channel amplitude ratio.
2. Background
In wireless communication systems, a base station typically transmits several physical channels. Physical channels may be separated in a variety of ways, including by frequency, time, or code, or by some combination of these. In Wideband Code Division Multiple Access (WCDMA) systems defined by the 3rd Generation Partnership Project (3GPP), numerous physical channels are employed. For example, some of the channels defined for the downlink (base-station-to-mobile transmissions) include the downlink dedicated physical channel (DPCH) for carrying user-specific data traffic, and the common pilot channel (CPICH), which is used by mobile stations to derive information about other channels (such as the DPCH) transmitted by a base station. 3GPP has also defined a High-Speed Physical Downlink Shared Channel (HS-PDSCH), which may carry data traffic targeted to several mobile stations.
A received signal differs from the transmitted signal in various ways due to the effects of the signal passing through the propagation channel, which includes the transmission medium (e.g. the air) as well as the analog components of the transmitting and receiving equipment. In wireless systems in particular, such effects on the radio-frequency signal include multipath fading, interference from other signals passing through the medium, and thermal noise. The signal received at the receiver can therefore be thought of as a composite signal consisting of a desired component and one or more impairment components representing distortion caused by the propagation channel, e.g., interference and noise.
Pilot symbols are often transmitted among information symbols on physical channels. These pilot symbols are used in mobile receivers for synchronization purposes and to derive information about the propagation environment experienced by the signals transmitted by the base station and received by the mobile receivers. This latter information may include propagation channel estimates, which model the effects of the propagation channel. These channel estimates are used by the receiver to reverse the effects of the channel and to reconstruct the transmitted symbols. Generally speaking, the more accurately the propagation channel is modeled in the receiver, then the more accurately the transmitted symbols can be reconstructed and decoded.
Channel estimation is well known in the art and is discussed, for example, in “Digital Communications” 4th Edition, by John G. Proakis, McGraw-Hill, 2000. In general, channel estimators compare received signal samples to “known” symbols to produce estimates of the channel response. These known symbols, or pilot symbols, may be derived from a physical channel specifically designated for carrying pilot symbols. For example, 3GPP has defined a common pilot channel (CPICH) which may be used by the mobile terminal as a source of pilot symbols for channel estimation. In general, pilot symbols may be transmitted on channels specifically designed to carry pilot symbols, or may be interleaved among information bits on a channel primarily intended for carrying data traffic. Although pilot symbols may be used for various purposes, the term “pilot symbol” as used herein refers to symbols used for channel estimation purposes, and “pilot channel” refers to a physical channel that carries pilot symbols.
In WCDMA systems, the CPICH is commonly used to provide a phase reference for the demodulation process, since the received CPICH signal is often stronger than other received signals and accurate channel estimates may be obtained from it. In some cases, propagation channel estimates derived from received CPICH symbols may be very useful in reconstructing and demodulating other symbols. For example, because the DPCH and the CPICH may be transmitted using much of the same radio frequency equipment at the base station, and from the same antenna, the two channels may experience essentially the same propagation response. In other words, the two channels will experience similar delays and phase changes, and will undergo similar multipath responses. Thus, it is recognized that channel estimates based on the received CPICH signals may be useful for estimating the channel response experienced by the DPCH signals.
However, the power of the CPICH will in many cases be much higher than the power of a traffic channel, such as the DPCH, transmitted by the same base station. Furthermore, the transmission powers for the traffic channel and the CPICH will in some cases differ by an amount unknown to the mobile, and may differ by amounts that vary over time. If pilot symbols are also available on the traffic channel, then the amplitude of the traffic channel response can be estimated by a channel estimator, and compared to the estimated pilot channel response to determine a ratio between the pilot channel amplitude and the traffic channel amplitude. In this case, pilot channel estimates may be scaled by this ratio to obtain estimates of the traffic channel response. Because the amplitude difference is accounted for, the resulting traffic channel estimates are largely independent of the pilot channel amplitude. This approach is described by U.S. Patent Application Publication 2007/0072552 A1.
However, other channels, such as the HS-PDSCH, may not carry pilot symbols from which the traffic channel amplitude can be easily estimated. In this case, the above technique is not practical.